Nitrated monoaromatic hydrocarbons (nitrophenols, nitrocatechols, nitrosalicylic acids) in ambient air: levels, mass size distributions and inhalation bioaccessibility

Nitroaromatic compounds in six major Chinese cities: Influence of different formation mechanisms on light absorption properties

Nitroaromatic compounds (NACs) are important nitrogen organics in aerosol with strong light-absorbing and chemically reactive properties. In this study, NACs in six Chinese megacities, including Harbin (HB), Beijing (BJ), Xi'an (XA), Wuhan (WH), Chengdu (CD), and Guangzhou (GZ), were investigated for understanding their sources, gas-particle partitioning, and impact on BrC absorption properties. The concentrations of ΣNACs in PM2.5 in the six cities ranged from 9.15 to 158.8 ng/m3 in winter and from 2.02 to 9.39 ng/m3 in summer. Nitro catechols (NCs), nitro phenols (NPs), and nitro salicylic acids (NSAs) are the main components in ΣNACs, with NCs being dominant in particulate phase and NPs being dominant in the gas phase. Correlation analysis between different pollutant species revealed that coal and biomass combustions were the major sources of NACs in the northern cities during wintertime, while secondary formation dominated NACs in the southern cities during summertime. The contribution of ΣNACs to brown carbon (BrC) light absorption ranged from 0.85 to 7.98 % during the wintertime and 2.07-6.44 % during the summertime. The mass absorption efficiency at 365 nm (MAE365) were highest for 4-nitrocatechol (4NC, 17.4-89.0 m2/g), 4-methyl-5-nitrocatechol (4M5NC, 15.0-76.9 m2/g), and 4-nitroguaiacol (4NG, 11.7-59.8 m2/g). The formation of NCs and NG through oxidation and nitration of catechol and guaiacol led to a significant increase in aerosol light absorption. In contrast, NPs and NSAs formed by the photonitration and photooxidation in liquid phase showed high polarity but low light absorption ability, and the proportions of (NPs + NSAs) in the light absorption of ΣNACs were lower than 15.3 % in the six megacities.

Toxicity of nitrophenolic pollutant 4-nitroguaiacol to terrestrial plants and comparison with its non-nitro analogue guaiacol (2-methoxyphenol)

Phenols, and especially their nitrated analogues, are ubiquitous pollutants and known carcinogens which have already been linked to forest decline. Although nitrophenols have been widely recognized as harmful to different aquatic and terrestrial organisms, we could not find any literature assessing their toxicity to terrestrial plants. Maize (monocot) and sunflower (dicot) were exposed to phenolic pollutants, guaiacol (GUA) and 4-nitroguaiacol (4NG), through a hydroponics system under controlled conditions in a growth chamber. Their acute physiological response was studied during a two-week root exposure to different concentrations of xenobiotics (0.1, 1.0, and 10 mM). The exposure visibly affected plant growth and the effect increased with increasing xenobiotic concentration. In general, 4NG affected plants more than GUA. Moreover, sunflower exhibited an adaptive response, especially to low and moderate GUA concentrations. The integrity of both plant species deteriorated during the exposure: biomass and photochemical pigment content were significantly reduced, which reflected in the poorer photochemical efficiency of photosystem II. Our results imply that 4NG is taken up by sunflower plants, where it could enter a lignin biosynthesis pathway.

Ultrafast Excited-State Proton Transfer in 4-Nitrocatechol: Implications for the Photochemistry of Nitrophenols

Nitrophenols are a class of environmental contaminants that exhibit strong absorption at atmospherically relevant wavelengths, prompting many studies of their photochemical degradation rates and mechanisms. Despite the importance of photochemical reactions of nitrophenols in the environment, the ultrafast processes in electronically excited nitrophenols are not well understood. Here, we present an experimental study of ultrafast electron dynamics in 4-nitrocatechol (4NC), a common product of biomass burning and fossil fuel combustion. The experiments are accompanied by time-dependent quantum mechanical calculations to help assign the observed transitions in static and transient absorption spectra and to estimate the rates of singlet-to-triplet intersystem crossing. Our results suggest that electronic triplet states are not efficiently populated upon 340 nm excitation, as efficient proton transfer occurs in the excited state on a time scale of a few picoseconds in water and tens of picoseconds in 2-propanol. This suggests that triplet states do not play a significant role in the photochemical reactions of 4NC in the environment and, by extension, in nitrophenols in general. Instead, consideration should be given to the idea that this class of molecules may serve as strong photoacids.

First insights into the molecular characteristics of atmospheric organic aerosols from Iasi, Romania: Behavior of biogenic versus anthropogenic contributions and potential implications

The present study reports first data on the organic molecular composition and evolution of secondary organic aerosols (SOAs) markers in aerosol samples from an urban environment in Romania. Targeted and non-targeted approaches of liquid chromatography tandem with time-of-flight mass spectrometry (LC-ToF-MS) were used as powerful analytical approaches for aerosol characterization at the molecular level. Four distinct organic molecular groups (CHO, CHON, CHONS, and CHOS) were classified as relevant for both warm (with 847 assigned molecular formulae) and cold (with 432 assigned molecular formulae) periods. Different formation mechanisms, physico-chemical processing, meteorological conditions, and sources origin or strengths (biogenic versus anthropogenic), were identified as governing factors of the mass concentration size distribution for the first generation and second-generation oxidation products of α-/β-pinene and two nitroaromatics (i.e., 4-nitrophenol and 4-nitrocatechol). Aromaticity equivalent (X_C), carbon oxidation state (OS_C), H/C and O/C ratios, and van Krevelen diagrams, were used to discriminate between: i) the aliphatic or aromatic nature of the identified organic aerosol constituents, ii) the oxidation state of the aerosol samples (e.g., more oxidized molecular formulae during the highly insolated period, more intense photochemistry), and iii) sources role in controlling OAs constituents abundances and behavior (e.g., higher relative contributions of aliphatic CHO formulae with a wider range of carbon numbers and CHOS molecular group with higher contribution during the warm period due to increased biogenic emissions or secondary formation from the biogenic precursors). Since in the present study >88 % of the 4-nitrocatechol and 4-nitrophenol was determined in the aerosol size fraction below 1 μm, it is believed that determination of their abundances and size distribution in ambient aerosols might provide direction for future studies such as to enhance the knowledge on their toxic potential levels for the human health.

Characterization of nitrated aromatic compounds in fine particles from Nanjing, China: Optical properties, source allocation, and secondary processes

Recently, nitrated aromatic compounds (NACs) have received much attention due to their role as key chromophores of brown carbon (BrC) and their impact on human health and the climate. In this study, a method for detection of 12 NACs in the atmosphere was developed and applied to the detection of 191 atmospheric samples in the northern suburbs of Nanjing in 2017. The average concentration of total NACs in Nanjing was 26.48 ng m^-3, which was lower than that in North China. The total NACs also showed obvious seasonal variation, with the highest concentration in winter (51.99 ng m^-3) and the lowest concentration in summer (11.26 ng m^-3). Moreover, the contribution of subcomponents of NACs also changed with the seasons. Nitrophenols (NPs) and nitrocatechols (NCs) were most abundant in winter, while nitrosalicylic acids (NSAs) were more abundant in summer, accounting for 30%, 27%, and 85%, respectively. The reason for this result may be due to the different sources of dominance of NACs in different seasons. The light absorption of NACs to water-soluble BrC was mainly concentrated in the 300-400 nm range, and its contribution reached the maximum at 310 nm. NPs and NCs had the highest contribution to BrC among all NACs in winter, with a range of 25-54% and 3-59%, respectively. The Positive Matrix Factorization (PMF) was used to analyze the main sources of NACs in different seasons. Secondary generation was the largest source in summer, accounting for 43.5%, and biomass combustion contributed the most in autumn, accounting for 36.7%. NACs are affected by temperature, especially in summer, and the subcomponents vary in temperature dependence. The secondary generation process of NACs is affected by NO₂ and O₃, especially when NO₂ is greater than 40 μg m^-3 and O₃ is less than 220 μg m^-3.

Seasonal variability of nitroaromatic compounds in ambient aerosols: Mass size distribution, possible sources and contribution to water-soluble brown carbon light absorption

Nitroaromatic compounds (NACs) as important constituents of atmospheric humic-like substances (HULIS) and brown carbon (BrC) affect the Earth's climate and pose a serious environmental hazard. We investigated seasonal size-segregated NACs in aerosol samples from the urban background environment in Ljubljana, Slovenia. Total concentrations of twenty NACs in PM_15.6 were on average from 0.51 ng m^-3 (summer) to 109 ng m^-3 (winter), and contributed the most to submicron aerosols (more than 74%). Besides 4-nitrocatechol (4NC) as the prevailing species, methylnitrocatechols (MNCs) and nitrophenols (NPs), we reported on some very rarely mentioned, but also on five novel NACs (i.e., 3H4NBA: 3-hydroxy-4-nitrobenzoic acid, 3MeO4NP: 3-methoxy-4-nitrophenol, 4Et5NC: 4-ethyl-5-nitrocatechol, 3Et5NC: 3-ethyl-5-nitrocatechol and 3MeO5NC: 3-methoxy-5-nitrocatechol). Concentrations of 3MeO5NC, 4Et5NC and 3Et5NC were enhanced during cold seasons, contributing up to 11% to total NAC in winter. In cold season, NAC size distributions were characterized with the peaks in the broader size range of 0.305-1.01 μm (accumulation mode), with 4NC and alkyl-nitrocatechols (∑(M/Et)NC) as the most abundant, followed by 4-nitrosyringol, nitrophenols and nitroguaiacols. In spring, a pronounced peak of ∑(M/Et)NC was observed in the accumulation mode (0.305-0.56 μm) as well as in the coarse one. A strong correlation of all NACs with ∑(M/Et)NC and levoglucosan indicates that primary emissions of wood burning were the most important source of NACs, but their secondary formation (e.g., aqueous-phase at higher ambient RH) in cold season could also be a significant one. In warmer season, NACs may be mostly derived from traffic-related aromatic VOCs. The contribution of NACs to the light absorption of the aqueous extracts was up to 10-times higher (contribution to Abs₃₆₅ up to 31%) than their mass contributions to WSOC (up to 3%) of corresponding size-segregated aerosols, confirming that most of the identified NACs are strong BrC chromophores.

Cytotoxicity and oxidative stress induced by atmospheric mono-nitrophenols in human lung cells

Nitrophenols (NPs) are hazardous pollutants found in various environmental matrices, including ambient fine particulate matter (PM_2.5), agricultural residues, rainwater, wildfires, and industrial wastes. This study showed for the first time the effect of three pure nitrophenols and their mixture on human lung cells to provide basic understanding of the NP influence on cell elements and processes. We identified NPs in ambient PM_2.5 and secondary organic aerosol (SOA) particles generated from the photooxidation of monocyclic aromatic hydrocarbons in the U.S. EPA smog chamber. We assessed the toxicity of identified NPs and their equimolar mixture in normal bronchial epithelial (BEAS-2B) and alveolar epithelial cancer (A549) lung cell lines. The inhibitory concentration-50 (IC₅₀) values were highest and lowest in BEAS-2B cells treated with 2-nitrophenol (2NP) and 4-nitrophenol (4NP), respectively, at 24 h of exposure. The lactate dehydrogenase (LDH) assay showed that 4NP, the most abundant NP we identified in PM_2.5, was the most cytotoxic NP examined in both cell lines. The annexin-V/fluorescein isothiocyanate (FITC) analysis showed that the populations of late apoptotic/necrotic BEAS-2B and A549 cells exposed to 3NP, 4NP, and NP equimolar mixture increased between 24 and 48 h. Cellular reactive oxygen species (ROS) buildup led to cellular death post exposure to 3NP, 4NP and the NP mixtures, while 2NP induced the lowest ROS buildup. An increased mitochondrial ROS signal following NP exposure occurred only in BEAS-2B cells. The tetramethylrhodamine, methyl ester, perchlorate (TMRM) assay showed that exposed cells exhibited collapse of the mitochondrial membrane potential. TMRM signals decreased significantly only in BEAS-2B cells, and most strongly with 4NP exposures. Our results suggest that acute atmospheric exposures to NPs may be toxic at high concentrations, but not at ambient PM_2.5 concentrations. Further chronic studies with NP and NP-containing PM_2.5 are warranted to assess their contribution to lung pathologies.

Molecular Characterization of Nitrogen-Containing Compounds in Humic-like Substances Emitted from Biomass Burning and Coal Combustion

N-containing organic compounds (NOCs) in humic-like substances (HULIS) emitted from biomass burning (BB) and coal combustion (CC) were characterized by ultrahigh-resolution mass spectrometry in the positive electrospray ionization mode. Our results indicate that NOCs include CHON+ and CHN+ groups, which are detected as a substantial fraction in both BB- and CC-derived HULIS, and suggest that not only BB but also CC is the potential important source of NOCs in the atmosphere. The CHON+ compounds mainly consist of reduced nitrogen compounds with other oxygenated functional groups, and straw- and coal-smoke HULIS exhibit a lower degree of oxidation than pine-smoke HULIS. In addition, the NOCs with higher N atoms (N₂ and/or N₃) generally bear higher modified aromaticity index (AI_mod) values and are mainly contained in BB HULIS, especially in straw-smoke HULIS, whereas the NOCs with a lower N atom (N₁) always have relatively lower AI_mod values and are the dominant NOCs in CC HULIS. These findings imply that the primary emission from CC may be a significant source of N₁ compounds, whereas high N number (e.g., N_2-3) compounds could be associated with burning of biomass materials. Further study is warranted to distinguish the NOCs from more sources.